All objects on Earth are subject to the Earth's own magnetic field. Magnetic fields are regions where invisible lines of force produced by permanent magnets and other magnetic sources are experienced or measured.

The magnetic fields associated with magnetic materials such as permanent magnets and the Earth's magnetic field are constant (non-alternating).

Magnetic fields can also be produced as a result of electric current passing through a conductor, This is known as Electromagnetism.

If the current is alternating (changing polarity, as in AC electricity), the resultant magnetic field is also alternating at the same frequency as the current producing it.

Low frequency magneticfields are usually difficult to shield against. Magnetic fields go through concrete, steel, and even solid lead with ease. There are some alloys (MU Metal for example) that can attenuate alternating magnetic fields.

It also possible to cancel them with another field of opposite phase or polarity. Video display terminal shields generally have NO effect on magnetic field component generated by these devices. The grounded VDT shields can affect only the electric field component.

Alternating magnetic fields have been shown to produce biological effects.